Devices using various types of media such as optical discs, magnetic optical discs, and flexible magnetic disk are known in the art as data storage devices. Among them, hard disk drives (hereinafter referred to as HDDs) have become popular as storage devices for computers, to such an extent that they are one of the storage devices indispensable for modern computer systems. Further, not limited to the computers as described above, HDDs are becoming more and more widely used in various applications. For example, HDDs are used for video recording/reproducing devices, car navigation systems, cellular phones, and removable memories for use in digital cameras.
A magnetic disk used in the HDD has a plurality of data tracks that are concentrically formed. A plurality of pieces of servo data each having address information, and a plurality of data sectors each including user data, are written to each data track. A plurality of data sectors are written between servo data areas. A desired data sector is accessed based on the address information of the servo data by use of a head element of a head slider supported by an actuator that is pivotally moved by a voice coil motor (VCM). This makes it possible to write data to the data sector or to read data from the data sector.
When the magnetic disk is being stopped, the actuator and the head slider are retracted to a retracted position. As a method for retracting the head slider, the load/unload method and the CSS (Contact Start Stop) method are known. For the load/unload method, the operation of moving a head from a ramp to a recording surface of a disk is called loading (head load), whereas the operation of retracting the head to a retracted position of the ramp is called unloading (head unload).
While the actuator is being moved between the retracted position and the recording surface of the magnetic disk, the HDD controls the pivotal movement of the actuator by controlling the speed of the VCM. To be more specific, a VCM speed detection circuit detects the VCM speed by use of the counter electromotive voltage of the VCM. A controller supplies the VCM with the VCM current so that a value of the detected VCM speed becomes a target value.
Here, during loading of the head, in order to avoid the damage to the head and the disk, the actuator is controlled so that the speed of the actuator is kept constant. For example, Japanese Patent Laid-Open No. 2005-293839 (“Patent Document 1”) discloses a storage device in which a correction coefficient used to convert the counter electromotive voltage into the correct moving speed is acquired for correction-coefficient calibration so that the moving speed detected by using the counter electromotive voltage coincides with the moving speed of an actuator, which is calculated from position information of a disk.
When the head is moved from the ramp to the disk, and consequently it becomes possible to stably read out position information, the load operation is completed. Then, the storage device starts seek operation of actually positioning the head to a target position on the disk. To be more specific, upon the completion of the load operation, it is possible to switch the speed control based on the counter electromotive voltage of the VCM to the positioning control based on servo information.
Incidentally, the damage may occur to the head and the disk in the HDD by various kinds of causes. As one of cause analyses, whether or not the load speed is faster than the target speed is detected to judge that the damage has occurred during the loading. If the load speed is faster than the target speed, the number of times of overspeeding and the overspeed are written to a specified area on the disk, the specified area being provided outside a data area to which user data is written. The number of times of overspeeding, the overspeed, and the like, which have been written, are used for failure analysis thereafter.
Until the head is loaded on the recording surface of the disk from the retraction area, it is not possible to read out position information written to the disk. In addition, unless it becomes possible to stably read out the position information, it is not possible to switch to the positioning control of the head based on the position information (servo information). For this reason, heretofore, for example, after it becomes possible to read out servo information, if servo data is successfully detected the specified number of times, switching to the control based on servo information is performed. Then, on the basis of the result of reading out servo data at the time of loading, the result being obtained before switching to the control based on servo information, the speed of the actuator is detected to perform abnormal speed detection.
However, even if an abnormal speed condition is detected on the basis of the result of reading out servo data at the time of loading, it is not possible to distinguish an abnormal speed condition caused by the damage to the disk or the head from an abnormal speed condition that is detected as a result of a failure to correctly read out servo data. Therefore, it is not possible to know whether information written as the abnormal speed condition is true or not.